ABC News' Abbie Boudreau and Brian O'Keefe report:

The workout plan that turned Henry Cavill into the "Man of Steel" was a super tough regimen that yields an amazing physique, but it's not just about exercise. It's also about eating a lot, according to his celebrity trainer, Mark Twight.

Twight trained both Cavill and Russell Crowe, who he says dropped 40 pounds for his part as Superman's father, Jor-El.

"What is the secret?" ABC News' Abbie Boudreau asked Twight.

"The secret is a well-kept secret called commitment and self-discipline," Twight laughed.

Twight says Cavill's transformation started by packing on 20 pounds, sticking to a strict 5,000 calorie intake.

"What was he eating?" Boudreau asked.

"We didn't care," Twight responded. "If he decided, 'Look, I can get an extra thousand calories by eating pizza, I'll do it.'"

Then the hard part came for the 30-year-old British actor. Cavill endured intense, two-hour workouts, five or six days a week, for a grueling 11 months.

"What we are trying to do is use physical means to basically prepare a person or change them in a psycho physical way so they can actually play a role in this context," said Twight. "So you need a guy to look like Superman? It will be better if he feels super in order to transmit that message.

"I would have to say that Henry turned into a man on this show. And not only in terms of physically, he made enormous changes, but in the way that he carried himself in relating and adapting to other people."